An example of an oil film bearing is the “MORGOIL” bearing, which is available from Siemens A G, Linz, Austria. The MORGOIL bearing has a hydrodynamic design which distributes the bearing load over a relatively large area. A film of oil on which the bearing operates has immense load carrying capacity and, when functioning as intended, eliminates wear caused by metal-to-metal contact. When used on a rolling stand, the oil film is constantly maintained by the hydrodynamic action of the rotating sleeve which fits over the roll neck, and a nonrotating bushing mounted in the chock.
A rolling mill oil film bearing typically comprises a sleeve axially received on and rotatably fixed to a roll neck. The sleeve is surrounded by a bushing contained in a chock mounted in a rolling mill housing. In service, the sleeve is rotatably supported on a thin film of oil hydrodynamically maintained at the bearing load zone between the sleeve and the bushing.
The sleeves may be either internally cylindrical for use on cylindrical or “straight” roll necks, or they may be internally tapered for use on tapered roll necks. Moreover, the sleeves may be “self locking” or “non-locking”. Self locking sleeves are frictionally fixed to the roll necks by interference fits, whereas non-locking sleeves require keys or the like to effect a mechanical inter-engagement with the roll necks.
With reference to FIG. 1, one such sleeve is shown at 13 as a component part of a rolling mill oil film bearing 12. The sleeve 13 is removably received on a tapered section 16 of the roll neck and is rotatably fixed to the roll neck by keys 14 or the like. The sleeve is surrounded by a bushing 18 and fixed in a bearing chock 20. In service, as previously noted, the sleeve is rotatably supported on a thin film of oil (not shown) hydrodynamically maintained at the bearing load zone between the sleeve and the bushing.
With reference to FIG. 2, it will be seen that the sleeve 13 has an internally tapered section 21 with a length L, an end section 22 extending axially beyond the outboard end of the internally tapered section, and a cylindrical outer surface 23 surrounding the internally tapered section. The cylindrical outer surface has a diameter D (also commonly referred to as the “journal” diameter). The internally tapered section has a taper angle a, a minimum thickness t at its inboard end, and a maximum thickness t′ at its outboard end adjacent to the end section 22. Keyways 15 co-act in mechanical interengagement with the keys 14, with both the keys and the keyways being located outside of the internally tapered section 21 and exclusively within the end section 22.
With reference to FIG. 3, it will be seen that a modern “MORGOIL” lubrication system delivers a controlled supply of oil to the bearings. When the roll is turning, the sleeve, fitted over the roll neck and secured by a key to prevent slippage, turns within the stationary bushing which is secured to the chock. Bearing load is distributed over the entire load zone and the continuity of the oil film is ensured by delivery of the oil through internal passages within the chock to the journal. Hydrostatics are used in oil film bearings when sustained operation under high load and low speed is required, when the mill is operating in a reversing mode, frequently changing the turning direction or when the mill is required to start under load after long stops. A high pressure pump feeds lubricant into the load zone of the bearings, assuring full film operation of the bearing under all rolling conditions.
FIG. 4 illustrates a cross-sectional view taken through a sleeve and bushing showing lubrication conduits, and ports for introducing and expelling lubricant.
To protect the bushing body against mechanical destruction from metal-to-metal contact with the sleeve, the bushing body is filled with so called white metal, which is a relatively soft metal providing emergency bearing characteristics. FIG. 5a is a section through the bushing 18 and shows that the quite ductile white metal filling 17 is supported by shoulders 19. With the continuing effort to reduce the size of the mills while increasing their output, the specific load capacity of bearings has increased as well. To meet this need MORGOIL introduced a High Strength Babbitt (HSB) white metal bearing material. This proprietary babbitt resulted in higher bearing capacity. FIG. 5b illustrates a MORGOIL bushing with HSB.
Significant limitations of the standard white metal bushings as well as the HSB bushings includes the risk of segregation between the steel bushing and white metal, high cost, and limited bearing capacity during emergency operating conditions, such as low oil pressure or non-stable oil film conditions.